Arylidene, or methine, dyes are of the formula ##STR1## wherein R, R.sup.1, and R.sup.2 are hydrogen atoms, alkyl or cycloalkyl groups, or substituted alkyl or cycloalkyl groups, R.sup.3 is a hydrogen atom, an alkyl or alkoxy group, a carbonylamino radical, or a fused phenyl ring, or R.sup.2 and R.sup.3 together form a ring, and E.sup.1 and E.sup.2 are strongly electron withdrawing unsaturated substituents.
Several methods are known for the preparation of arylidene dyes. In one of these methods an aromatic amine is formylated with a mixture of phosphoryl chloride and dimethylformamide in a Vilsmeier reaction. A description of this reaction is given in K. Venkataraman, The Chemistry of Synthetic Dyes, Academic Press, N.Y., 1970, Vol. III, p. 450. The aminosubstituted benzaldehyde produced in this step is next condensed with an active methylene compound to yield an arylidene dye.
This method of preparing arylidene dyes has severe limitations in that many aromatic amines undergo formylation in the Vilsmeier reaction poorly, with undesirable side reactions. For example, if aromatic amines have hydroxy or acetamido substituents, such groups undergo chlorination by the phosphorus oxychloride employed in the reaction. Similarly, anilines containing only one N-alkyl substituent yield undesirable side products under the conditions of the reaction.
U.S. Pat. No. 4,006,178 discloses the preparation of arylidene dyes containing dicyanovinyl substituents ##STR2## where R denotes a hydrogen atom or an alkyl, cycloalkyl, or aralkyl group. Dicyanovinyl substituted arylidene dyes are prepared by the reaction of an aromatic amine with a 1-halogeno-2,2-dicyanoethylene, preferably 1-chloro-2,2-dicyanoethylene. This latter compound can be prepared by several multistep procedures. One method, described in U.S. Pat. No. 2,774,783, starts with 1-acetoxy-1,1-dicyanoethane, which is pyrolyzed to yield 1,1-dicyanoethylene, which, without isolation, is chlorinated to 1,2-dichloro-1,1-dicyanoethane, which product is subjected to a second pyrolysis step to give 1-chloro-2,2-dicyanoethylene.
This process for the preparation of dicyanosubstituted arylidene dyes suffers from the disadvantage that the required reactant 1-chloro-2,2-dicyanoethylene must be prepared and isolated from a multistep process. Furthermore, in its reaction with an aromatic amine, hydrogen chloride is formed. This requires either that a 100% excess of the aromatic amine be employed to neutralize the generated acid or that a stoichiometric quantity of a more strongly basic amine, for example, triethylamine, be added under closely controlled conditions to neutralize the acid.